kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 30 insertions, 0 deletions

diff --git a/include/drm/drm_utils.h b/include/drm/drm_utils.h
new file mode 100644
index 000000000..7e077484c
--- /dev/null
+++ b/include/drm/drm_utils.h
@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: MIT */
+/*
+ * Function prototypes for misc. drm utility functions.
+ * Specifically this file is for function prototypes for functions which
+ * may also be used outside of drm code (e.g. in fbdev drivers).
+ *
+ * Copyright (C) 2017 Hans de Goede <hdegoede@redhat.com>
+ */
+
+#ifndef __DRM_UTILS_H__
+#define __DRM_UTILS_H__
+
+#include <linux/types.h>
+
+struct drm_edid;
+
+int drm_get_panel_orientation_quirk(int width, int height);
+
+struct drm_panel_backlight_quirk {
+	u16 min_brightness;
+	u32 brightness_mask;
+	bool force_pwm;
+};
+
+const struct drm_panel_backlight_quirk *
+drm_get_panel_backlight_quirk(const struct drm_edid *edid);
+
+signed long drm_timeout_abs_to_jiffies(int64_t timeout_nsec);
+
+#endif